As a positive electrode for the non-aqueous electrolyte secondary battery, there has been proposed a lithium-transition metal oxide. As negative electrodes for the non-aqueous electrolyte secondary battery, there has been proposed graphite, amorphous carbon, oxide, lithium alloy and metallic lithium. Nowadays, lithium cobaltate (LiCoO2) is mainly used as a positive active material. However, this active material is expensive. Accordingly, in order to cope with the expected huge consumption of non-aqueous electrolyte secondary batteries, development of less expensive positive active materials is important.
Further, from the view point of the increasing interest in environmental issues, it is quite desirable that the active material is as environmentally friendly as possible. Nowadays, compounds containing manganese, nickel or iron are under extensive studies as the positive active materials for the non-aqueous electrolyte secondary battery. Among these elements, iron is the most inexpensive and environmentally friendly material. Thus, an iron compound is very attractive as a positive active material for the next-generation non-aqueous electrolyte secondary battery.
Various iron compounds have been heretofore proposed as iron-containing positive active materials for the non-aqueous electrolyte secondary battery. Among these iron compounds, iron oxyhydroxide (FeOOH), which is an active material delivering high capacity, has recently attracted much attention. Among several types of FeOOH with different crystalline structure, β-FeOOH has a relatively large size of tunnel structure of (2×2). It is expected that the use of β-FeOOH as a positive active material for the non-aqueous electrolyte secondary battery would provide good battery properties.
Amine et al. investigated the positive-electrode characteristics in the secondary lithium cells using high-crystalline β-FeOOH as a positive material. They reported that this active material exhibited an excellent charge-discharge characteristics at a low current density of 0.1 mA/cm2 (J. Power Sources, 81-82, 221 (1999)). However, the conventional high-crystalline β-FeOOH has a drawback of poor cycle-life performance at a higher current density.
An object of the present invention is to provide an inexpensive and environmentally friendly non-aqueous electrolyte secondary battery by using an unknown novel iron compound as a positive active material.